How do I make a model (in specific, a car) lock when you sit in the seat?

Lock all the parts when someone sits down.

local car = workspace.Car; --You may have to edit this
local seat = car.VehicleSeat; --And this
local children = car:GetChildren();

local lockCar = function(locked)
    for key = 1, #children do
        local value = children[key];
        if value:IsA("BasePart") then
            value.Locked = locked;
        end
    end
end;

seat.ChildAdded:connect(function() lockCar(true); end);
seat.ChildRemoved:connect(function() lockCar(false); end);

Every time someone sits down on a seat, a child is added in to Seat. It is called SeatWeld. When they get off, SeatWeld is removed. You will need to fill in some of the userdata where I require it, as I don't know what exactly is where.

local car = -- Guessing it's in workspace somewhere. Just give the userdata here.
local seat = car.Seat -- Guessing seat is in the model car. (PLEASE make the car a model if it isn't already.)

function RunToggle(val) -- Will be called later.
    for i,v in pairs(car:getChildren()) do
        if v:IsA("BasePart") then
            c.Locked = val -- Idk why you want it to be locked, I'm guessing you might mean that the car doors no longer open, in which case you would need to have every brick you want closed in your car named Door.
            if v.Name == "Door" then
                v.CanCollide = val
            end
        end
    end
end

seat.ChildAdded:connect(function(c)
    if c:IsA("Weld") then
        RunToggle(true)
    end
end)
-- Below is to cancel everything done if the person gets up.
seat.ChildRemoved:connect(function(c)
    if c:IsA("Weld") then
        RunToggle(false)
    end
end)